Microstructure and mechanical properties of in situ Al–Mg2Si composites

Abstract

In situ metal matrix composites (MMCs) with Mg₂Si particulate reinforcement have been developed recently as ultralight materials. In this paper, a brief overview of the physical and mechanical properties of Mg₂Si and the current status of research on Mg₂Si reinforced MMCs is presented, followed by more detailed information on recent progress in the research group of the present authors. The effects of element additions and processing parameters on the microstructure of the composites obtained by gravity casting are discussed, together with some mechanical property data.     

High temperature oxidation behavior of a Mo–3Si–1B(wt%) alloy

Abstract

Molybdenum base alloys with the addition of small amounts of silicon (2–4.5 wt.%) and boron (～1 wt.%) can form a passivating layer protecting the alloy from further rapid oxidation. When such molybdenum base alloys are exposed to oxidizing environments at high temperatures, a borosilicate glass layer can form that will reduce the transport of oxygen to the alloy to limit further oxidation. Oxidation is then controlled by diffusion through the borosilicate glass layer. The focus of this research was to study the mechanisms and kinetics of high temperature oxidation of a Mo–Si–B alloy. The base alloy has a composition of Mo–3Si–1B (wt.%) and was studied in a variety of gas environments over a range of temperatures in order to elucidate the critical factors that allow it to develop a protective borosilicate glass layer. The borosilicate glass layer is protective when no continuous channels exist in the layer extending from the gas interface to the alloy interface. The borosilicate layer is believed to contain channels in the early stages of development and the elimination of the channels is obtained by appropriate control of the temperature and gas flow conditions whereby MoO₃ is removed via vaporization while the borosilicate viscosity is not increased due to loss of B₂O₃. Once the borosilicate layer is continuous and free of channels, subsequent oxidation occurs by inward diffusion of oxygen and the outward diffusion of molybdenum through this layer with vaporization of MoO₃ occurring at the gas/borosilicate layer interface, and MoO₂ and additional borosilicate forming at the alloy/MoO₂ interface.     

Tensile and fatigue behaviour of Ni–Ni3Si eutectic in situ composites

A Ni–Ni₃Si composite was fabricated via a eutectic reaction (Ni–Ni₃Si) using a rapidly cooled directional solidification technique at a solidification rate of 40?μm?s^?1. The composite consisted of approximately 62.2% Ni–Si solid solution and 37.8% Ni–Ni₃Si eutectic phase in volume. Four-point bend fatigue tests were carried out on the composite. The fatigue strength of the alloy was measured to be 520?MPa (maximum cyclic stress). It was found that the fatigue cracks were preferably initiated in the Ni–Ni₃Si eutectic phase, and that the Ni matrix was fractured in a cleavage fashion. It was probably attributed to the high level of supersaturated Si in the Ni matrix, which led to inducing the embrittlement of the Ni matrix.     

Low temperature sintering and properties of Ca–Al–B–Si–O glass/ceramic composites with various ceramic fillers

Low-temperature sintering and properties of low temperature co-fired ceramics materials based on a typical Ca–Al–B–Si–O glass and various ceramic fillers such as (Zr_0.8Sn_0.2)TiO₄, (Ca_0.5Mg_0.5)TiO₃, BaSm₂Ti₄O₁₂ and CaTiO₃ were investigated. Densification, crystallization and dielectric properties are found to strongly depend on the type of filler. The densification process of glass/ceramic composites with different ceramic fillers is mainly from 600 to 925 °C, and the initial compacting temperature of samples is 600 °C. The initial rapid densification of samples starts after glass softening temperature of samples. The XRD patterns of (Ca_0.5Mg_0.5)TiO₃ and CaTiO₃ samples demonstrate crystalline phases, CaTiO(SiO₄) and CaTiSiO₅, respectively, as a result of firing at 875 °C for 15 min. The high dielectric constant fillers produce high ε_r values of the dielectric samples. The maximum dielectric constant of samples for (Zr_0.8Sn_0.2)TiO₄, (Ca_0.5Mg_0.5)TiO₃, BaSm₂Ti₄O₁₂ and CaTiO₃ filler is 14.02, 16.21, 18.64 and 23.78, respectively. Comparing with other samples, the specimens for (Ca_0.5Mg_0.5)TiO₃ and CaTiO₃ ceramic filler have lower dielectric loss. Especially, the sample for (Ca_0.5Mg_0.5)TiO₃ filler exhibits the lowest dielectric loss of 0.00011.     

Microstructural stability, microhardness and oxidation behaviour of in situ reinforced Ti–8.5Al–1B–1Si (wt%)

Microstructural stability, microhardness and oxidation behaviour of an in situ reinforced Ti–8.5Al–1B–1Si (wt%) alloy was examined in both air and argon environments. When exposed for up to 5760 min at temperatures below the /+2 transius, hardening occurred in both air and argon environments. The increase in hardness in the air heat-treated samples is attributed to a combination of solid-solution strengthening due to the oxygen and the precipitation of the 2 phase, while the increase in hardness in the argon heat-treated samples is primarily due to the precipitation of the 2 phase. On the other hand, when heat treated above the /+2 transius, after an initial increase in hardness there is a drop in hardness which is attributed due to elimination of the 2 phase and a decreased contribution of boron and silicon in the matrix towards the solid-solution strengthening by virtue of coarsening of the TiSi2 and TiB reinforcements. Oxidation of the alloys follows a parabolic oxidation law when oxidized both in an environment of flowing air and static air with the primary oxidation product being TiO2. The activation energy for oxidation is 200 kJ mol-1 in an environment of flowing air and 303 kJ mol-1 in static air. The difference in activation energy arises from the difference in the availability of oxygen at the reaction front in the two cases. © 1998 Chapman & Hall     

Mechanical properties and oxidation resistance of C–B–Si coated silicon nitride fiber reinforced Si–N–C composites with cross-ply structure

To enhance the oxidation resistance of a ceramic matrix composite, a C–B–Si interface layer was applied between the fiber and the matrix. The layer was deposited on the fiber by chemical vapor deposition. Three types of coatings were prepared: A1, A2 (multilayers of graphite layer/B–C–Si crystalline layer/graphite layer) and B1 (monolayer of B and C containing graphite). The multilayer coated CMC retained 88–97% of the original strengths after oxidation at 1523 K for 36 ks. The monolayer coated CMC degraded to 55% of its original strength after oxidation, but had a high fracture toughness (28 MPa m^1/2) before oxidation. The differences of the oxidation resistance and fracture toughness were discussed in relation to the microstructure of the coatings.     

Synthesis and properties analysis of char-reinforced Al–13.5Si–2.5Mg alloy composites

The re-evaluation of previous and existing methods in materials processing is becoming ever more critical because of processing and starting materials cost factors. A study on the synthesis and properties investigation of hypereutectic Al–13.5Si–2.5Mg alloy reinforced with carbon chars using coconut shell as the organic precursor has been carried out. The low-cost, double compaction solid-state technique was used. Reinforcing the hypereutectic alloy with coconut shell char particles (size:<140 m) at 2 vol % and consolidating by reaction sintering at 600 °C in vacuum for 15 min, followed by near net-shape compaction at 250 MPa, increased the hardness of the alloy 6% while reducing its strength (UTS) by only 3%. The use of palm kernel shell char as the dispersed phase was found to yield identical results. At 2 vol % char, the mechanical properties, sintered density and dimensional changes were optimally found to be suitable for lightweight anti-friction electromechanical applications. Attempts to reinforce the alloy with 2 vol % coconut shell chars activated in CO2 reduced its strength in the range of 19 to 26% at different burn-off percentages. This is attributed to the higher amount of oxide products formed during the activation process. At 600 °C, formation of the brittle Al4C3 phase in the different sintered composites containing activated and unactivated chars was identified by X-ray studies. © 1998 Chapman & Hall     

Microstructure and tensile properties of graded Al–Mg2 Si in situ composites fabricated by centrifugal casting

Abstract

Al–Mg₂ Si in situ composite tubes were fabricated by a centrifugal casting process. The microstructure and tensile strength of the composite tubes were examined revealing an inhomogeneous distribution of Mg₂ Si particles along the radial direction. Adjacent to the rapidly cooled area, near the outer periphery, in situ particles were forced inwards leaving behind a particle free region in the middle part of the tubes. Increasing Mg₂ Si weight percentage in the aluminium matrix induced dendrite formation in the rapidly cooled area and the width of the particle free region in the middle part of the cylinder wall decreased. The particle distribution gradient in the inner region also decreased with increasing Mg₂ Si content. The width of the particle free region and the particle distribution gradient were both influenced by the gradually varying viscosity of the liquid alloy as it solidified. The tensile strength of the centrifugally cast tubes increased from inner to outer periphery. High volume fractions of Mg₂ Si particles of between 50 and 70 vol.-% reduced the strength of the composites.     

Initial oxidation of Fe–Cr alloys: in situ STM and ex situ SEM observation

Abstract

In order to understand the initial oxidation of Fe–Cr alloys a single crystal of Fe–15Cr (100) was oxidized at 440°C under controlled oxygen partial pressure in a UHV system and the surface morphology was observed using in situ STM (basic pressure 1×10^?10 mbar); in addition, polycrystalline Fe&15Cr was oxidized at 400°C in an IR-furnace in atmospheric air and the morphology was observed using ex situ SEM. The chemistry of the surface oxide layers was studied by XPS.

Preparation of the single crystal in the UHV system did not lead to segregation of Cr to the surface during heating. In situ STM investigation showed that oxidation of Fe–Cr commenced by nucleation of Cr oxide on the surface, due to selective oxidation of Cr. When the Cr at the surface and at the interface was completely consumed by nucleation of Cr oxide, Fe oxidized and covered the initial Cr oxide nuclei, resulting in an Fe oxide layer on the surface. Ex situ experiments showed that initial oxidation of the mechanically prepared polycrystalline alloy depended on the defect distribution in the surface. It started with formation of whisker-type Fe oxides along defects and proceeded with spherical-type nucleation and growth of Fe oxide. In both experiments, the final product on the surface was Fe₂O₃.     

Thermal conductivity of Cu–Zr/diamond composites produced by high temperature–high pressure method

Copper matrix composites reinforced with about 90 vol.% of diamond particles, with the addition of zirconium to copper matrix, were prepared by a high temperature–high pressure method. The Zr content was varied from 0 to 2.0 wt.% to investigate the effect on interfacial microstructure and thermal conductivity of the Cu–Zr/diamond composites. The highest thermal conductivity of 677 W m⁻¹ K⁻¹ was achieved for the composite with 1.0 wt.% Zr addition, which is 64% higher than that of the composite without Zr addition. This improvement is attributed to the formation of ZrC at the interface between copper and diamond. The variation of thermal conductivity of the composites was correlated to the evolution of interfacial microstructure with increasing Zr content.     

Synthesis and characterization of barium borosilicate glass–Al2O3 composites

Barium borosilicate glass with composition 30BaO–60B₂O₃–10SiO₂ glass was prepared by melt-quenching technique. Different weight % of crystalline Al₂O₃ was mixed with the glass powder and sintered at optimum temperature. The changes in the structure and thermal properties of the glass with alumina content were investigated by X-ray powder diffraction, FT-IR spectroscopy and differential thermal analysis. The variations in the coefficient of thermal expansion and dielectric properties with composition were also studied and correlated with the structural changes.     

Synthesis and characterization of chitosan–carbon nanotube composites

Acid functionalized single walled carbon nanotubes were covalently grafted to chitosan by first reacting the oxidized carbon nanotubes with thionyl chloride to form acyl-chlorinated carbon nanotubes which are subsequently dispersed in chitosan and covalently grated to form composite material, CNT–chitosan, 1, which was washed several times to remove un-reacted materials. This composite has been characterized by FTIR, 13C NMR, TGA, SEM and TEM and has been shown to exhibit enhanced thermal stability. The reaction of 1, with poly lactic acid has also been accomplished to yield CNTchitosan–g-poly(LA), 2 and fully characterized by the above techniques. Results showed covalent attachment of chitosan and chitosan–poly lactic acid to the carbon nanotubes.     

Sintering, densification and crystallization of Ca–Al–B–Si–O glass/Al2O3 composites for LTCC application

Ca–Al–B–Si–O glass/Al₂O₃ composites were prepared based on the borosilicate glass powders (D₅₀ = 2.84) and Al₂O₃ ceramic powders (D₅₀ = 3.26), and the sintering, densification, crystallization of samples were investigated. The shrinkage of sample starts to have a sharp increase at 600 °C. The shrinkage of sample starts to have a further rapid increase after the glass softening temperature of about 713 °C. Glass/Al₂O₃ composites can be sintered at 875 °C/15 min and exhibit better properties of a relative density of 98.4 %, a λ value of 2.89 W/mK, a ε _r value of 7.82 and a tan δ value of 5.3 × 10^?4. The interface between glass and Al₂O₃ grains and the interface between anorthite and glass phase depicts a good compatibility according to transmission electron microcopy test. It is the low sintering temperature, high density and good compatibility with Ag electrodes that, guarantee borosilicate glass/Al₂O₃ composites suitable for low temperature co-fired ceramic materials.     

Selective laser melting of in situ TiN–Ti5Si3 composites with RE–Si–Fe addition: comparative study

Abstract

A comparative study was conducted to investigate the influence of rare earth (RE)–Si–Fe addition on the microstructures and tribological properties of in situ TiN–Ti₅Si₃ composites prepared by selective laser melting (SLM). With the addition of 3 wt-% RE–Si–Fe, the shape of in situ TiN reinforcing particles became rounded and smoothened, the particle size was refined and the dispersion state was homogenised. Relative to TiN–Ti₅Si₃ composites without RE addition, the average friction coefficient of SLM processed RE containing composites decreased from 0·85 to 0·55, and the resultant wear rate decreased from 3·79×10^?4 to 2·65×10^?4 mm³ N^?1m^?1. Metallurgical functions of the RE elements in improving the SLM processability and the resultant wear performance of in situ composites were elucidated.     

High temperature strength,fracture toughness and oxidation resistance of Nb–Si–Al–Ti multiphase alloys

Nb–Si–Al–Ti quaternary phase diagram around three-phase region, which consists of niobium solid solution (Nb_ss), Nb₃Al and Nb₅Si₃, is constructed in this study. The three-phase region exists up to titanium content of about 20 mol%. Based on the quaternary phase diagram, three-phase alloys containing Nb_ss from about 50 to 75% in volume are prepared to improve high temperature strength, room temperature fracture toughness and oxidation resistance simultaneously.When microstructure and composition are optimized (Nb_ss+Nb₃Al+Nb₅Si₃) three-phase alloy with the addition of titanium exhibits higher compressive strength than nickel-based superalloys at room temperature to 1573 K. Fracture toughness at room temperature of (Nb_ss+Nb₃Al+Nb₅Si₃) three-phase alloys is increased to over 12 MPa m^1/2 by the addition of titanium without sacrificing high temperature strength. Oxidation resistance of (Nb_ss+Nb₃Al+Nb₅Si₃) three-phase alloys is improved by the addition of titanium.     

High temperature strength,fracture toughness and oxidation resistance of Nb–Si–Al–Ti multiphase alloys

Nb–Si–Al–Ti quaternary phase diagram around three-phase region, which consists of niobium solid solution (Nb_ss), Nb₃Al and Nb₅Si₃,is constructed in this study. The three-phase region exists up to titanium content of about 20 mol%. Based on the quaternary phase diagram, three-phase alloys containing Nb_ss from about 50 to 75% in volume are prepared to improve high temperature strength, room temperature fracture toughness and oxidation resistance simultaneously.

When microstructure and composition are optimized (Nb_ss + Nb₃Al + Nb₅Si₃) three-phase alloy with the addition of titanium exhibits higher compressive strength than nickel-based superalloys at room temperature to 1573 K. Fracture toughness at room temperature of (Nb_ss + Nb₃Al + Nb₅Si₃) three-phase alloys is increased to over 12 MPa m^1/2 by the addition of titanium without sacrificing high temperature strength. Oxidation resistance of (Nb_ss + Nb₃Al + Nb₅Si₃) three-phase alloys is improved by the addition of titanium.     

The role of oxidation in time-dependent response of ceramic–matrix composites

In this work, a model is constructed to account for the effect of oxidation of the fiber, fiber interface coating and surrounding matrix on the stress distribution and strain accumulation in ceramic–matrix composites. The model includes the role of the fabric architecture, the effect of porosity and the distribution of cracks in its formulation and utilizes oxidation rate constants and phenomenological models for the progress of oxidation as reported in literature.Dwell fatigue experiments were carried out for silicon carbide/silicon carbide nitride (SiC/SiNC) and Melt infiltrated silicon carbide/silicon carbide (MI SiC/SiC) composites to evaluate their time-dependent strain accumulation. Strain accumulation due to oxidation calculated by the model was compared to time-dependent strain obtained from experiment and showed that the rate of strain accumulation due to oxidation was low before the fibers were exposed to the environment but drastically increased after that. Such high rate of strain accumulation can be one of the main causes for failure of the composite.Model results showed that strain accumulation in both composites due to oxidation was dependent on the stress level with the SiC/SiNC accumulating more strain at similar stress levels. This can be explained by the higher modulus of the MI SiC/SiC that limits deformation, reducing crack density and accordingly decreasing the chance of oxygen to infiltrate the specimen and oxidize the fibers. Strain accumulation due to oxidation was also dependent on the fabric architecture and stress distribution within the unit cell. Additionally, comparing the effect of the value of the linear and parabolic oxidation rate constants reported by different researchers showed that not only is their absolute value important, but also their ratio to one another.